1. Field of the Invention
The present invention relates to the construction of concrete products. In particular the present invention relates to devices for forming holes in hollow concrete manhole casings or concrete jars for receiving steps to enable workmen to enter and exit the interior of the manhole casings or concrete jars.
2. Description of the Related Art
In many construction projects such as roads, sewers, and large buildings, concrete manhole casings are installed to provide access to underground sewers, and tunnels containing water and steam pipes and electrical cables. Such concrete manholes are constructed from concrete reinforced with steel and may be circular or square in cross-section. Commonly, the diameter of such concrete manholes may range from three feet to six feet and the height is selected to extend from the surface of the ground downward to the depth of the tunnel to which access is desired.
Many such concrete manhole casings or concrete jars are pre-cast at a location remote from the construction site and shipped to the site after casting and curing. Such reinforced concrete manhole casings are sometimes referred to in the art and herein as jars or concrete jars.
Commonly, after concrete is poured into a mold to form a jar, after the concrete hardens to the point where the mold can be removed, concrete drills are used to drill holes in the interior walls of the jar to receive steps to enable a workman to enter and exit the jar. Such drilling is slow and difficult, and therefore adds significantly to the cost of the jar. Since the jar is reinforce with steel rod or wire, the drill sometimes strikes the steel and increases the difficulty of drilling holes for receipt of steps.
Exemplary of the related art are the following patents: U.S. Pat. Nos. 5,653,078; 5,174,910; 4,655,318; 4,365,780; 4,241,543; 4,195,709; 4,084,780; 1,093,204; 890,573; Canadian Patent 1,107,543; and Japanese Patent 60019833.